FIG. 7 is a flow diagram illustrating the normal steps involved in the manufacture of a mirror-surface wafer of the prior art. With reference to the diagram, a general description will be given of a normal method for the manufacture of a mirror-surface wafer employed as a raw material wafer for the production of a semiconductor devices.
First, a single crystal ingot is grown by means of the Czochralski method (CZ method) or the floating zone melting method (FZ method) or the like (STEP 101). Because of distortions (warpage) in the peripheral shape of the grown single crystal ingot, the periphery of the ingot is ground by a cylindrical grinding machine or the like in an outer shape grinding step (STEP 102) to adjust the peripheral shape of the ingot. The ingot is sliced using a wire saw or the like in a slice step (STEP 103) to produce a disc-shaped wafer of thickness of the order of 500 to 1000 μm, and the periphery of the wafer is then further chamfered in a chamfering step (STEP 104).
Following this, the wafer is flattened by planar grinding and/or lapping or the like (STEP 105), and a chemical polishing process is administered thereon in an etching step (STEP 106). Furthermore, coarse polishing (STEP 107) and a final polishing (STEP 108) are implemented on the wafer surface, after which a wafer washing (STEP 109) is implemented to produce a mirror-surface wafer.
A very high level of flatness has been demanded in the production of high-precision devices in recent years for the production of semiconductor devices in which circuits are formed on the surface of mirror-surface wafers obtained by way of these steps. A low level of wafer surface flatness generates a problem whereby, because of the partial lack of focus of the lens focal point that occurs during exposure in the photolithography step, the formation of the minute patterns of a circuit is difficult. In addition, the flattening of the surfaces of not only semiconductor wafers but also other target materials for polishing comprising a flat surface such as liquid crystal substrates is demanded.
For the manufacture of a wafer with a very high level of flatness such as this the polishing of the wafer is regarded as extremely important. An example of a well-known general polishing apparatus for implementing this polishing is an apparatus that comprises a disc-shaped polishing plate to which an abrasive cloth is affixed to the upper surface and a wafer chuck for holding one surface of the wafer to be polished and pushing the other surface of the wafer against the abrasive cloth, the polishing being implemented by the supplying of a slurry between the wafer and the abrasive cloth and the relative rotation of the wafer and the polishing plate.
In addition, because the abrasive cloth is elastic, when polishing is implemented with the wafer only pushed against the abrasive cloth, the wafer embeds slightly into the abrasive cloth. When this happens, because of the concentration of elastic stresses from the abrasive cloth on the edge of the wafer, the pressure applied to the wafer is larger at the peripheral part than the center part and results in the excess polishing of the peripheral part of the wafer.
Apparatuses to alleviate this problem are available in which abrasive cloth deformation on the peripheral part of the wafer is suppressed so as to prevent excess polishing by the concentric arrangement of a toroidal presser ring with the periphery of the wafer chuck, and the pushing of the abrasive cloth by the presser ring at the desired pressure. An example thereof is the polishing apparatus disclosed in U.S. Pat. No. 6,350,346 as shown in FIG. 8. In this polishing apparatus a presser ring 52 is provided on the outer side of a wafer chuck 51, the wafer chuck 51 and the presser ring 52 can be relatively rotated, and the pressure force of each can be independently controlled. In addition, the presser ring 52 can be moved vertically with respect to a top ring 53.
However, in actual practice the production of a presser ring 52 that is perfectly parallel to the abrasive cloth 54 is very difficult. Notably, because only the presser ring 52 can be moved vertically in this constitution, the presser ring 52 and the abrasive cloth 54 are not formed perfectly in parallel and a distribution of the pressure generated at the pressing ring surface occurs during polishing which, accordingly, sometimes results in a worsening of the level of flatness of the wafer edge part worsens and the production of a polished wafer of an asymmetric shape.